Removing a blog reference from a tag

I'm working on an application in Node with Mongoose where you're able to post blog entries and tag them. When a blog entry is deleted, I want to remove it's reference from the blog, and here's where I need help.

Below is the route for deleting a blog entry, but I get "TypeError: Cannot call method 'find' of undefined" when I try to delete a blog entry, so I guess my code below is wrong.

app.post('/blog/delete/:id', function(req, res){

    model.BlogPost.findById(req.params.id, function (err, blog){
        if (err) {
            console.log(err);
            // do something
        }

        blog.remove(function(err) {
            console.log(err);
            // do something
        });


        var query = model.Tag.find( { blogs: { $in : blog } } );

        query.exec(function (err, tags) {
            if (err) {
                console.log(err);
                // do something
            }
            tags.remove();

            res.redirect('back');
        });
    });
});

Model for blog entries:

var BlogPostSchema = new Schema({
    name : String,
    type : String,
    author : ObjectId,
    title   : String,
    body : String,
    buf : Buffer,
    date: { type: Date, default: Date.now },
    comments : [CommentSchema],
    meta : {
        upvotes : Number,
        downvotes : Number,
        // points : { type Number, default: },
        favs : Number,
        uniqueIPs : [String],
        tags : [String]
    }
});

modelObject.BlogPost = mongoose.model('BlogPost', BlogPostSchema);

Model for tags:

var TagSchema = new Schema({
      name              : String
    , blogs             : [String]
});

modelObject.TagSchema = TagSchema;
modelObject.Tag = mongoose.model('Tag', TagSchema);